Reference Guide

848 | Virtual Link Trunking (VLT)
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VLT domain - This domain includes both VLT peer devices, the VLT interconnect, and all of the port
channels in the VLT connected to the attached devices. It is also associated to the configuration mode that
must be used to assign VLT global parameters.
VLT peer device - One of a pair of devices that are connected with the special port channel known as the
VLT interconnect (VLTi).
Configuring Virtual Link Trunking
VLT requires that you enable the feature and then configure the VLT domain, backup link, and VLT
interconnect on both peer switches.
Important Points to Remember
• VLT port channel interfaces must be switch ports.
• If RSTP is included on the system, it must be configured before VLT. See RSTP Configuration.
• Dell Force10 strongly recommends that the VLTi (VLT interconnect) must be a static LAG and that
LACP should be disabled on the VLTi.
• The spanning tree root bridge should be at the Aggregation layer. If RSTP is enabled on the VLT
device, refer to RSTP and VLT for guidelines to avoid traffic loss.
• If both VLT peers are rebooted in JumpStart mode and VLT LAGs are static, the DHCP server reply to
the DHCP discover offer may not be forwarded by the ToR to the correct node. To avoid this scenario,
configure the VLT LAGs to the ToR and the ToR port channel to the VLT peers with LACP. If
supported by the ToR, enable the
lacp-ungroup feature on the ToR using the command lacp ungroup
member-independent port-channel.
• If the lacp-ungroup feature is not supported on the ToR, VLT peers should be rebooted one at a time.
After rebooting, verify that VLTi (ICL) is active before attempting DHCP connectivity.
• When IGMP snooping is enabled on the VLT peers, ensure the value of the
delay-restore command is
not less than the query interval.
• When Layer 3 routing protocols are enabled on VLT peers, make sure the
delay-restore timer is set to a
value that allows sufficient time for all routes to establish adjacency and exchange all the L3 routes
between the VLT peers before the VLT ports are enabled.
•The
lacp ungroup member-independent command should only be used if the system connects to nodes
using BMP to upgrade or boot from the network.
• Ensure all port channels where LACP ungroup is applicable are configured as hybrid ports and as
untagged members of a VLAN. BMP uses untagged DHCP packets to communicate with the DHCP
server.
• If the DHCP server is located on the ToR and the VLTi (ICL) is down due to a failed link when a VLT
node is rebooted in JumpStart mode, it will not be able to reach the DHCP server, resulting in BMP
failure.
Configuration Notes
When you configure VLT, the following conditions apply: